Power converters are used to synthesize a desired voltage from an available voltage using pulse width modulation techniques. In the most general case, the synthesized voltage has a different frequency and amplitude in relation to the incoming voltage (e.g. variable speed motor drives.) In such applications, the power converter of choice is either an alternating current (AC)-direct current (DC)-AC rectifier-inverter system or an AC-AC matrix converter system. The common characteristic of all these systems is the need for an energy storage element or source (equivalent in electrical terms) that provides an outer boundary/envelope within which a desired voltage synthesis is done.
The simplest form of such a converter is a buck converter, where a desired output voltage Vo is synthesized from a source voltage Vs, using fixed frequency pulse wave modulation (PWM) techniques, with the relation, Vo=D×Vs, where D is the duty cycle of operation for the switch. Vo falls within the envelope defined by Vs and zero volts. FIG. 1A shows a conventional inverter. With FIG. 1A's inverter, the ‘envelope’ is defined by the two DC supply voltages. The desired output voltage falls within this envelope if it is to be successfully synthesized. This technique forms the basis for control for all DC/DC converters, inverters, and voltage source converters. FIG. 1B shows a region of achievable output voltage corresponding to the conventional inverter shown in FIG. 1A.
In the case where multi-phase AC sources are available, a desired output voltage (DC or AC) could be synthesized directly from the multiple AC sources using a conventional ‘matrix converter’. FIG. 2A shows a schematic of a single output line of a conventional matrix converter. FIG. 2B shows an ‘envelope’ within which a desired output voltage is to be synthesized using the conventional matrix converter of FIG. 2A. Both, inverters and matrix converters, are able to synthesize any frequency on the output. While it may appear that matrix converters do not require bulk energy storage in DC capacitors, they do equivalently need additional sources, as well as switches to interconnect each available phase to the desired output terminal.
Power system networks are currently faced with the problem of the inability to control voltages and currents in the network. In any meshed power network, control of the voltage magnitude and phase angle is of vital importance owing to the constant increase in the load on the network as well as the erratic nature of load profiles. Conventional methods of control include shunt VAR compensators, shunt and series FACTS devices and phase angle regulators. FACTS devices are faced with the problem of high cost and have thereby not significantly penetrated the area of power flow control. Phase angle regulators are in use extensively however, they provide slow response and only phase angle control. The need for a device that provides phase angle control and well as voltage amplitude control simultaneously is highlighted by the inadequacies of current technologies.
Furthermore, power networks require voltage amplitude control within a certain range, roughly at ±10% of the nominal voltage. Therefore, a solution to the problem of control of voltages and currents is required that provides an adequate level of control on the system while having a minimal topology with minimum cost.